Upon combustion of inhomogeneous or contaminated fuels, in particular, such as household refuse, flue gases are formed, which, besides the main combustion products of carbon dioxide and water, to an increasing degree of contamination, also contain numerous contaminants, such as dust, acidic gases, heavy metals and organic substances, for example. In this context, it is known that many of these groups of contaminants, in particular, however, the heavy metals released during combustion, become concentrated in the fly ash.
In the course of a waste-gas purification, the fly ash, together with a multiplicity of the contaminants bound into the same, are separated out as filter ash or filter dust by dust separators immediately following the combustion and flue gas cooling. At present, filter ash or filter dust from waste incineration plants is dumped in underground landfills since it is not suited for an open landfilling (for example, in accordance with waste class II, TA household waste) due, inter alia, to the high concentration of water-soluble metal compounds. Therefore, for an open landfilling, it is desirable that the fly ash be rendered harmless prior to a landfilling of this kind.
Christine Hallgren, Birgitta Strömberg: Current Methods to Detoxify Fly Ash from Waste Incineration; Report no. TPS 2004:1 Svensk Fjärrvärme AB, 2004, ISSN 1402-5191, describes currently available technologies for treating filter ash from waste incineration plants that have already been tested in the industry or are operated as pilot plants. In this respect, the following methods for separating heavy metals from fly ash are known:
What is generally referred to as the CT-Fluapur process is a thermal process in which the filtered-out heavy metal-containing fly ash is introduced as filter ash into a hydrochloric acid atmosphere at 900° C. In the hydrochloric acid atmosphere that is present, heavy metals or compounds are converted into volatile metal chlorides. In this context, these heavy metal chlorides partially evaporate, the heavy metal content in the remaining fly ash to be landfilled thereby decreasing. The released gaseous metal chlorides then react with water vapor, forming solid metal oxides, and can subsequently be separated off from the gas stream.
The Christine Hallgren et al. report referenced above also discusses a wet chemical method that utilizes what is generally known as the 3R process. It provides for an acidic extraction of heavy metals from filter dust; after approximately 15 minutes reaction time in an acid atmosphere (pH value approximately 1), up to 89% cadmium, 68% zinc, 18% copper and 22% lead being extracted from the filter dust. The remaining solid residues of the filter dust are subsequently solidified together with a binding agent and returned again to the combustion process to destroy the organic contaminants (for example, PCDD, PCDF) bound into the same. The extracted heavy metals may be separated out from the aqueous solution and fed to a recycling.
The Christine Hallgren et al. report likewise discusses what is generally referred to as the MR process. It provides in a first step for an alkaline extraction, in particular of sulfates (for, example, K, Na) at pH values of between 9 and 12. This is followed by an acidic extraction using hydrochloric acid to dissolve heavy metals, a separation of residue and dissolved heavy metals, as well as a liberation of residues from salts by washing with water. The remaining filter ash is subsequently heated in a rotary furnace for about one hour at a temperature of over 600° C. to destroy organic contaminants such as PCDD and PCDF.
To date, cost considerations have precluded an industrial scale use of all of the mentioned methods and, instead, direct underground disposal (i.e., without additional treatment) has been favored.